1. Field of the Invention
A regenerating braking system for motor vehicles controls regenerative brake torque acting on wheels so as to improve fuel efficiency and ensure stability and controllability of a vehicle. Such a regenerative braking system is disclosed in Japanese patent laid-open publication Nos. 2000-050409, 2000-270409, and (Tokkaihei) 05-161209.
2. Description of the Related Art
A regenerating braking system for motor vehicles controls regenerative brake torque acting on wheels based on a so as to improve fuel efficiency and ensure stability and controllability of a vehicle. Such a regenerative braking system is disclosed in Japanese patents laid-open publication Nos. 2000-050409, 2000-270409, and (Tokkaihei) 05-161209.
Japanese patent laid-open publication No. 2000-050409 discloses a regenerative braking system (a first conventional system) having an electric generator connected with front wheels. This first system is capable of applying regenerative brake torque to the front wheels in addition to brake torque due to master cylinder fluid pressure, but prohibits the generator from applying the regenerative brake torque under the condition that estimated vehicle body speed is a predetermined-value larger than the maximum wheel speed between front left wheel speed and front right wheel speed. Under this condition, the brake torque applied to the wheels is obtained only by brake fluid pressure, not by the generator, for avoiding excessive understeer to ensure stability and controllability of a vehicle.
Japanese patent laid-open publication No. 2000-270409 discloses a regenerative braking system (a second conventional system) having an electric generator connected with rear wheels. This second system is capable of applying regenerative brake torque to the rear wheels in addition to brake torque caused by master cylinder fluid pressure, and also shifting regenerative braking control modes of the generator from a normal-road control mode to a low-friction road control mode when a low-friction road is detected. By this mode shift, the regenerative brake torque is reduced for avoiding excessive oversteer to ensure stability and controllability of a vehicle.
Japanese patent laid-open publication No. (Tokkaihei) 05-161209 discloses a regenerative braking system (a third conventional system) having an electric generator connected with rear wheels. This third system is capable of applying regenerative brake torque to the rear wheels in addition to brake torque caused by master cylinder fluid pressure, and also shifting regenerative braking control modes from a regenerative-braking priority control mode to a normal control mode when a steering angle exceeds a predetermined angle and vehicle speed exceeds predetermined speed. By this mode shift, application of the regenerative brake torque is prohibited and only brake torque due to master cylinder fluid pressure is applied under obtaining an ideal brake force distribution onto front wheels and rear wheels for avoiding excessive oversteer to ensure stability and controllability of a vehicle.
The above known conventional regenerative braking systems, however, encounter a problem that improvement in fuel efficiency and improvement in stability and controllability of a vehicle are incompatible with each other.
The first conventional system loses a margin for recoverable regenerative energy, resulting in degradation in fuel efficiency, because it prohibits application of the regenerative brake torque in a not-understeer range, for example a neutral range where the vehicle runs with a large steering angle and with road grip.
The second conventional system might cause excessive oversteer on the medium or high friction road, resulting in degradation in stability and/or controllability of the vehicle, because it does not reduce the regenerative brake torque even when excessive oversteer causes during the vehicle runs on a medium or high friction road.
The third conventional system degrades fuel efficiency, because it always reduces the regenerative brake torque under the condition that a steering angle exceeds the predetermined angle and vehicle speed exceeds the predetermined speed, even when oversteer does not occur.
In addition to the above problem, the conventional regenerative braking systems can not judge an occurrence of understeer during regenerative braking with accuracy, which brings difficulty in optimum control of the regenerative braking. In order to control the regenerative braking optimally, an understeer amount during regenerative braking needs to be detected quickly from its weak understeer state.
The occurrence of understeer is conventionally judged based on a variation between actual yaw rate and target yaw rate by using a yaw rate sensor. Such technology is described in Japanese patent laid-open publication No. (Tokkaihei) 07-223520. However, the yaw rate sensor has bad detection accuracy in a small yaw rate range, and is not suitable for a use of controlling the regenerative braking. It has large time lag in detection of an understeer tendency of the vehicle, since it detects yaw rate from yawing movement of its vehicle body after a front left and right wheel speed variation reflects on behavior of the vehicle. Besides, the sensor is sensitive to not only yaw component caused by movement of the vehicle body, but also various components except the yaw component, which degrades accuracy in yaw rate detection of the sensor.
It is, therefore, an object of the present invention to provide a generative braking system for motor vehicles which overcomes the foregoing drawbacks and can detect an understeer amount with accuracy and ensure compatibility between improvement in fuel efficiency and improvement in stability and/or operability of a vehicle by using the accurate understeer amount.